Containers for packaging food require a combination of physical properties which is not economically available with rigid and semi-rigid containers made from any single polymeric material. Among the properties required are low oxygen and moisture permeability, compatibility with the temperatures and pressures encountered in conventional food processing and sterilization, and the impact resistance and rigidity required to withstand shipping, warehousing, and abuse. Multi-layer constructions comprised of more than one plastic material can offer such a combination of properties.
Multi-layer containers have been made commercially by thermoforming and extrusion blow molding processes. These processes however, suffer from major disadvantages. The chief disadvantage is that only a portion of the multi-layer material formed goes into the actual container. The remainder of the material can sometimes be recovered and used either in other applications or in one of the layers of future containers made by the same process. This "recycle" use, however, recovers only a part of the value of the original material because the scrap is a mixture of the materials. Other disadvantages of these processes include limited options in terminal end geometry or "finish," in shape, and in material distribution.
Injection molding and injection blow molding are often preferred for making single layer containers because they are scrapless and overcome many of the other limitations of thermoforming and extrusion blow molding. These processes have not been commercially adapted to multi-layer constructions because of difficulties in achieving the required control of the location and uniformity of the various layers, particularly on a multi-cavity basis. In fact, even on a single cavity basis, multi-layer injection molding has been limited to relatively thick parts in which a thin surface layer of plastic covers a relatively thick core layer of either foamed plastic or of some other aesthetically unattractive material such as scrap plastic.
To be successfully commercially adapted to food containers, multi-layer injection molding would require two major improvements over the processes which are now commercially practiced. Economical multi-layer food containers require very thin core layers comprised of relatively expensive barrier resin such as a copolymer comprised of vinyl alcohol and ethylene monomer units. The location and continuity of these thin core layers are important and must be precisely controlled. U.S. patent applications, Ser. No. 059,375, now abandoned in favor of Continuation Ser. No. 324,824, and Ser. No. 059,374, each assigned to the assignee of this application and incorporated herein by reference, disclose multi-layer, injection molded and injection blow molded articles, parisons and containers having a thin continuous core layer substantially encapsulated within inner and outer structural layers, and methods and apparatus to make them. The disclosures in the aforementioned applications apply to both single and multi-cavity injection molding machines.
The second improvement over current commercial multi-layer injection molding processes is that the process must be capable of forming containers on a multi-cavity basis. Although the relatively large parts made by current commercial multi-layer processes can be economically practiced on a single cavity basis, food containers, which are relatively small, require a multi-cavity process to be economical. The extension from single cavity processes to an acceptable multi-cavity process presents many serious technical difficulties.
One way to extend from a single cavity to a multi-cavity process would be to replicate for each cavity the polymeric material melting and displacement and other flow distributing means used in a single cavity process. Such replication would realize some advantages over a unit cavity process. For example, a common clamp means could be used. However, it would not provide the maximum advantage because individual polymeric material melting and displacement means would still be necessary. Such a multiplicity of melting and pressurization means would not only be costly but would create severe geometrical and design problems of positioning a large number of separate flow streams in a balanced configuration, thereby increasing the required spacing between cavities, and limiting the number of cavities which would fit within the area of the clamped platens.
An alternate means of molding multi-layer articles on a multi-cavity basis would be to have a single multi-layer nozzle with its associated melting, displacement and distributing means communicate with a single channel or runner feeding multiple materials to multiple cavities. Such a runner system might be either of the cold runner type in which the plastic in the runner is cooled and removed with the injection molded article in each cycle, or of the hot runner type in which the plastic remaining in the runner after each shot is kept hot and is injected into the cavities during subsequent shots. The chief limitation of this single runner approach is that the single runner channel itself would contain multiple materials which would make it very difficult to control the flow of the individual materials into each cavity, particularly for a process having elements of both sequential and simultaneous flow such as that described in U.S. patent application Ser. No. 059,374. Controlling the flow of multiple materials in a single runner would be even more difficult in a case in which the runner is long, as in a multi-cavity system.
In the preferred embodiments of the apparatus and methods of this invention, a single displacement source is used for each material which is to form a layer of the article, but the materials are kept separate while each material is split into several streams each feeding a separate nozzle for each cavity. The individual materials are thereby combined into a multi-layer stream only at the individual nozzles, in their central channels, which feed directly into each cavity. Although this approach avoids many of the disadvantages of the previously described methods, it presents many problems which must be satisfactorily overcome for successful injection of articles in which thin core layers are properly distributed and located.
Several of these problems result from the length of the runner and the distribution system for a multi-coinjection nozzle machine. For economical reasons, it is desirable to have as many cavities as possible within the machine in order to provide as many articles as possible upon each injection cycle. It is possible to minimize the average runner length for a given number of cavities by having the channels run directly to the remotest nozzle, redirecting a part of the stream as it passes near each other nozzle. It has been found that such a channel geometry, while suitable for most single layer injection molding, has a major disadvantage for precise multi-layer injection in that a given impetus introduced at the displacement or pressurization source will have its effect more immediately in the more proximate nozzles than in the more remote ones. The time delay between the initiation of an impetus and its effect at a distance results from the compressibility of the plastic. Because of this compressibility, material must flow in the channel before a desired pressure change can be achieved at a remote location. It has been found that in order to achieve the same flow initiation and termination times and the same relative flow rates of various layers in each nozzle as well as to obtain articles from all cavities having substantially the same characteristics, the material entering each nozzle must have undergone essentially the same flow experience in its path to the nozzle.
It has further been found that in a system in which a given flow stream is split into several individual streams to feed each nozzle, the channel and device geometries which accomplish each of these flow splittings must be symmetrically designed so as to provide the same flow experience to the material in each of the resulting split streams. Such symmetry is difficult to achieve with viscoelastic materials such as polymer melts because the materials have a "memory" of their previous history. When a flow channel contains a sharp turn, for example, material which has passed near the inner radius of curvature of that turn will have a different flow experience from the material which has passed near the outer radius of curvature.
Even with a runner system which, by its design, minimizes the differences in flow history in the path to each nozzle, there will remain some differences as a result of remaining memory effects, temperature non-uniformities in the melt stream before it is split, temperature non-uniformities in the runner system, and machining tolerances. For this reason, it would be desirable to have independent control of the time of initiation and termination of each flow, a critical requirement for precise control of thin core multi-layer injection molding. Such independent control should be effected as near as possible to the point at which the individual flow streams are combined into a multi-layer flow stream. Although these control means should be located in each individual nozzle, they should be controlled in such a manner that they are actuated simultaneously in desired nozzles of a multi-coinjection nozzle machine.
It is not sufficient that the flow of each material be substantially identical in each nozzle. It is also necessary that the flow of the individual materials be uniformly distributed within each injection cavity and, hence, within the nozzle channel feeding the cavity. For axisymmetrical articles, such as most food containers, this is most readily achieved by shaping the various flow streams into concentric annular flows or by shaping one stream into a cylindrical flow and shaping the other flows into annular flows concentric with that cylinder before combining the flow streams.
In order to achieve the required uniformity in these concentric annular flows, it is necessary to redistribute a given flow stream from its shape as it leaves the runner system into a balanced annular flow. Achieving such a balanced annular flow is difficult in itself but is much more difficult to achieve with an intermittent flow process than it is, say, in conventional blown film dies where the flow is constant. Among the complexities of such an intermittent flow process are the difficulty of achieving flow balance when the rate of flow is deliberately varied during each cycle, and the additional problem of different time response behavior at various locations around the annulus.
An additional requirement for an acceptable multi-cavity, multi-layer runner system is that it accurately align and maintain an effective pressure contact seal between each nozzle with its respective cavity. This alignment is particularly critical for the injection of the internal layer of the multi-layer articles in that any misalignment will adversely affect the uniformity and location of the internal layer. The difficulty in achieving such alignment is that the metal for such a hot runner system is at a higher temperature than is the metal plate in which the cavities are mounted. Because of the thermal expansion of materials of construction normally used for such mold parts, the nozzle to nozzle distance will tend to grow with temperature more than will the cavity to cavity distance. In single layer, multi-cavity injection molding, there are two conventional ways of compensating for this difference in thermal expansion. The first is to prevent the relative expansion or contraction by physical restraint; that is, by physically interlocking the runner with the cavity plate. For a large runner system, such a physical constraint system will generate large often problematical opposing forces in the two parts. The second way is to size the runner system so that it will align with the cavity plate when it is at an elevated temperature within a narrow range, even though it will be misaligned beyond the range, e.g., at room temperature. In accordance with this invention, the runner system is not attached to the cavity plate, but rather is left free to grow radially. The nozzles and cavity faces are flat to provide a sliding interface. Given this feature, and that the cavity sprue orifices are provided with a larger diameter than that of the nozzle sprue orifices, the runner has a much greater opportunity to grow radially without the cavity and nozzle sprue orifices becoming misaligned. This provides a much broader temperature range within which to operate, and a wider range of possible polymer melt materials which can be used. However, in order for the nozzles mounted in the runner to transfer plastic at high pressure to the cavities without leakage, it is necessary to impose an opposing force to counteract the separation force generated by this high pressure. This is conventionally achieved by transmitting all or part of the force of the injection clamp through the runner system to the fixed platen. An alternative method is, to use the axial thermal expansion of the runner system to generate a compressive force on the runner between the fixed platen and the cavity plate. One difficulty with any of the above methods of compensating for this differential expansion is that they require close physical contact between the hot runner and the colder metal of the cavity plate and of the fixed platen. This close contact causes thermal variations in the runner. While such thermal gradients would be acceptable in a single layer runner system, the resulting differences in flow experience to each nozzle could for example result in a significant variation in the uniformity and location of a thin inner layer in multi-layer injection molding. This invention overcomes these problems by mounting the runner system with minimum contact between it and surrounding structure.
Other problems encountered in multi-cavity injection molding of articles relates to the formation of high-barrier multi-layer plastic containers. Such containers require that the leading edge of the internal barrier layer material be extended substantially uniformly into and about the marginal end portion of the side wall of the parison or container. This condition is difficult to obtain, because of the compressibility of polymeric melt materials and the long runners of multi-cavity machines which result in a delay in flow response which is accentuated the more remote the materials are from the sources of material displacement. In addition, there are the previously mentioned difficulties of achieving balanced annular flow and uniform time response due for example to variations in polymer and machine temperatures and in machining tolerances, and due to the intermittency of the flow process. These factors render it difficult to introduce a polymeric melt material uniformly and simultaneously over all points of its orifice in one co-injection nozzle, and likewise with respect to introducing the corresponding material through corresponding orifices in the plurality of co-injection nozzles. It has been found that such an introduction is important to extending the leading edge uniformly into the marginal end portion of a container side wall because the portion of the annulus of material first introduced into the central channel will first reach the marginal end portion of the parison or container side wall in the cavity, while the last introduced portion will trail and may not reach the marginal end portion. This condition, referred to as "time bias," has been found to be one cause of bias in the leading edge of the internal layer, which is unacceptable for, for example, quality, high oxygen barrier containers for highly oxygen sensitive food products.
Another problem is that even if the internal layer material is introduced without time bias into the central channel, there may still be bias in the leading edge of the internal layer material in the side wall of the injected article, if all portions of the annulus of the leading edge of the internal layer material are not introduced into or onto a flow stream in the central channel having a substantially uniform velocity about its circumference. This is difficult to achieve for one reason because the flow stream having a substantially uniform velocity about its circumference is not necessarily radially uniform. If this type of introduction occurs, therre will be what is referred to as "velocity bias" in that the portions of the annulus in the central channel introduced onto a flow stream which has a high velocity will reach the marginal end portion of the side wall of the article in the cavity before those portions of the annulus introduced onto a flow stream having a lower velocity. Thus, in such case, other things being equal, even though there was no time bias in the introduction of the annulus of the internal layer material, a velocity bias in the central channel and cavity nevertheless resulted in a biased leading edge in the marginal end portion of the side wall of the injected article.
These and other problems associated with multi-layer unit and multi-coinjection nozzle injection molding and injection blow molding machines, processes and articles are overcome by the apparatus, methods and articles of this invention.
Accordingly, it is an object of this invention to provide methods and apparatus for commercially injection molding multi-layer, substantially rigid plastic parisons and containers, and for commercially injection blow molding multi-layer, substantially rigid plastic articles and containers by means of multi-cavity, co-injection nozzle machines.
It is another object of this invention to provide the above methods and apparatus for so molding said items by means of multi-cavity, multi-coinjection nozzle machines.
Another object of this invention is to provide and commercially manufacture, at high speeds, injection molded and injection blow molded, thin, substantially rigid, multi-layer, plastic articles, parisons, and containers.
Another object of this invention is to provide the above methods and apparatus for manufacturing the aforementioned articles, parisons and containers on a multi-cavity multi-coinjection nozzle basis, such that each item injected into and formed in each cavity has substantially identical characteristics.
Another object is to provide injection molding and blow molding methods and apparatus which overcome problems of long runners, variations in temperatures within structural components, variations in temperatures and characteristics of individual and corresponding polymer melts, and variations in machining tolerances which may occur with respect to multi-layer multi-cavity machines.
Another object of this invention is to provide methods and apparatus for providing a substantially equal flow path and experience for each corresponding polymer material flow stream displaced to each corresponding passageway of each co-injection nozzle for forming a corresponding layer of an aforementioned item to be injected.
Another object of this invention is to provide methods and apparatus for preventing bias in the leading edge of the internal layer in the marginal edge portions of the previously mentioned articles, and in the marginal end portion of the side walls of the above-mentioned articles, parisons and containers.
Another object of this invention is to provide methods and apparatus for forming such articles, parisons and containers wherein the leading edges of their internal layers are substantially uniformly extended into and about their marginal edge portions and the marginal end portions of their side walls.
Another object of this invention is to provide method for positioning, controlling and for utilizing foldover of a portion of the marginal end portion of said internal layer or layers to reduce or eliminate bias and obtain said substantially uniformly extended leading edge of the internal layer or layers.
Another object is to provide methods of avoiding and overcoming time bias and velocity bias as causes of biased leading edges in articles formed by injection molding machines and processes.
Another object is to provide methods of pressurizing polymer melt materials in their passageways to improve their time responses, provide greater control over their flows, obtain substantially simultaneous and uniform onset flow of their melt streams substantially uniformly over all points of their respective nozzle orifices, and obtain substantially simultaneous and identical time responses and flows of corresponding melt streams of the materials in and through each of the multiplicity co-injection nozzles of multi-cavity injection molding and blow molding machines.
Another object is to provide separate valve means operative in the central channel of a co-injection nozzle to there block and unblock the nozzle orifices in various desired combinations and sequences, to control the flow and non-flow of the polymer melt materials through their orifices.
Another object is to provide the aforementioned valve means wherein they are commonly driven to be substantially simultaneously and substantially identically affected in each co-injection nozzle of a multi-coinjection nozzle injection molding machine.
Another object of this invention is to control the relative locations and thicknesses of the layers, particularly the internal layer(s) of the previously mentioned multi-layer injection molded or injection blow molded items.
Another object of this invention is to provide methods and apparatus for obtaining effective control of the polymer flow streams which are to form the respective layers of the injected items, in the passageways, orifices and combining areas of co-injection nozzles and in the injection cavities of multi-cavity injection molding and blow molding machines.
Another object of this invention is to provide co-injection nozzle means adapted to provide in co-injection nozzles, a controlled multi-layer melt material flow stream of thin, annular layers substantially uniformly radially distributed about a substantially radially uniform core flow stream.
Another object of this invention is to provide runner means for a multi-cavity, multi-coinjection nozzle injection molding machine, which splits each flow stream which is to form a layer of each injected item, into a plurality of branched flow streams, and directs each branched flow stream along substantially equal paths to each co-injection nozzle.
Yet another object of this invention is to provide the aforementioned runner means which includes a polymer flow stream redirecting and feeding device associated with each co-injection nozzle for redirecting the path of each branched flow stream for forming a layer of the item to be injected, and feeding them in a staggered pattern of streams to each co-injection nozzle.
Still another object is to provide apparatus for multi-layer, multi-coinjection nozzle injection molding machines, including floating runner means and a force compensation system, for compensating for injection back pressure and maintaining an on-line effective pressure contact seal between all co-injection nozzles and all cavities of the machines.
The foregoing and other objects, features and advantages of this invention will be further appreciated from the following description and the accompanying drawings.